The center of Greek life was the domestic hearth which was also the sacrifical altar. Hestia, as Goddess of the Hearth, also reigned over personal security and happiness and the sacred duty of hospitality. Hestia is also credited with the invention of building houses.
In mathematics, the graph of a function f is the collection of all ordered pairs (x, f(x)). If the function input x is a scalar, the graph is a two-dimensional graph, and for a continuous function is a curve. If the function input x is an ordered pair (x1, x2) of real numbers, the graph is the collection of all ordered triples (x1, x2, f(x1, x2)), and for a continuous function is a surface.
The graph of a function $f$ is concave upward at the point $(c,f(c))$ if $f'(c)$ exists and if for all $x$ in some open interval containing $c$, the point $(x,f(x))$ on the graph of $f$ lies above the corresponding point on the graph of the tangent line to $f$ at $c$.